1. Field of the Invention
The present invention relates to a suckback method capable of positively effecting a suckback operation in an injection molding machine which employs a servomotor as an injection axis drive source for axially driving the screw.
2. Description of the Related Art
In metering operation, when a screw receives the back pressure and at the same time is drivingly rotated, resin material in a heating cylinder becomes molten by virtue of thermal energy supplied from a heater and frictional heat generated during screw rotation. As an amount of the molten resin accumulated in the distal end portion of the heating cylinder increases, the screw is subjected to molten resin pressure. This pressure is in the opposite direction of the screw movement and would cause the screw to be moved backward. Thereafter, the screw rotation is stopped when the screw reaches a metering completion position. Then, the socalled suckback operation is effected by slightly moving the screw backward and at the same time eliminating the back pressure so as to disconnect the molten resin material between a sprue bushing and a nozzle to prevent the resin from loosely falling off. An injection molding machine of a type equipped with a hydraulic drive source on its injection axis can easily effect such suckback operation. On the other hand, in an injection molding machine of a different type employing a servomotor as the injection axis drive source and limiting the output torque of the servomotor to provide a controlled back pressure during the metering process, the following drawback can occur. That is, when the torque limit for the servomotor is released to generate a drive force enough to effect suckback operation, the screw is moved forwardly by virtue of the presence of an error amount accumulated in an error register of a servo circuit for driving the servomotor, and hence the suckback operation cannot be smoothly performed.